docs: display v2 design + plan (pluggable scanout backend)
Keep the GOP framebuffer as the floor, make scanout a pluggable backend, and upgrade to a native virtio-gpu driver when it announces itself (dynamic hot-attach; GOP stays the fallback for "no driver ever"). v2 builds the shm cross-process memory capability, shared with the future client-surface path. Milestones V1 (backend seam) → V6 (resilience + tests), each with a gate.
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Daniel Samson
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# Display v2 — build plan (pluggable scanout: GOP floor + virtio-gpu native)
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The ordered, checkpointable build-out for [display-v2.md](display-v2.md). Each milestone
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lands on its own and ends in a **verifiable gate** — shaped for a `/loop` run, like
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[display-plan.md](display-plan.md). Read display-v2.md first for the *why*.
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## Locked decisions (do not relitigate)
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- **First native backend = virtio-gpu** (VM standard: mode-set + present/flush + vsync).
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- **Dynamic hot-attach**: boot on GOP, upgrade to native when the driver **announces**
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(push, not polling); re-attach across driver restarts; GOP is the floor for "no driver
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ever," not a live fall-back after a reprogram.
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- **v2 builds the `shm` capability** (endpoints → memory objects), shared with the future
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client-surface path.
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- The compositor's layers/back-buffer/damage are **unchanged**; only scanout is pluggable.
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## Conventions
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Follow [coding-standards.md](coding-standards.md): spell out non-acronym abbreviations,
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kebab-case file names, no `Co-Authored-By` trailers. New user binaries go through
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`addUserBinary` and get packed into the initial-ramdisk; protocols are
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`b.addModule("…-protocol", …)` imported into `runtime`; new syscalls extend
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[abi.zig](../system/abi.zig) `SystemCall` + a `library/runtime` wrapper.
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## How to verify along the way
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- `zig build test` — host unit tests (backend selection logic, virtio structs/encodings).
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- `python3 test/qemu_test.py <case>` — boots the kernel in QEMU; assert on serial markers.
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The virtio path boots with `-device virtio-gpu` (a new per-case `qemu_extra`).
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- The `run-x86-64` target renders to a window for visual confirmation.
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---
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## V1 — The scanout backend seam (refactor, no behaviour change)
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Extract scanout from the compositor so today's path becomes one backend among future ones.
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- [ ] A `Backend` interface in `system/services/display/`: `surface() -> {ptr, pitch,
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format, width, height}`, `present(damage: Rect)`, and capability flags
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(`canModeSet`, `hasVsync`, both false for now).
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- [ ] Wrap the v1 GOP path as `GopBackend` (claim the `display` node, WC-map the LFB,
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`present` = the current damage-rect WC copy). The compositor composes into
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`backend.surface()` and calls `backend.present(damage)` — no direct LFB references
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left in the compositor core.
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- [ ] Pure backend-selection logic factored so it's host-testable.
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**Gate:** `display-service` + `display-demo` still pass unchanged (pure refactor; GOP is
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the only backend), and `zig build test` stays green.
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## V2 — The `shm` cross-process memory capability (kernel)
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- [ ] [abi.zig](../system/abi.zig): `shm_create`, `shm_map` syscalls (+ a `ServiceId`/cap
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convention if needed). Kernel handlers: `shm_create(len)` allocates page-aligned RAM,
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returns a handle + maps it; passing the handle as an `ipc_call` `send_cap` shares it;
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`shm_map(cap)` maps the same physical pages into the receiver. Reclaimed on death.
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- [ ] `library/runtime/shm.zig` (+ barrel export): `create(len) -> Region{handle, ptr}`,
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`map(cap) -> ptr`.
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- [ ] Reuse the M13 capability-passing machinery (endpoints → memory objects).
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**Gate:** a kernel/qemu `shm` test — process A `shm_create`s a region, writes a pattern,
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passes the cap to process B, which `shm_map`s it and reads the same bytes back (proving
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shared physical pages, not a copy). Heartbeat `shm: shared N bytes ok`.
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## V3 — The virtio-gpu driver: bring-up + a frame on screen
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- [ ] `system/drivers/virtio-gpu/`: claim the virtio-gpu PCI function (device-manager
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match on its PCI/virtio id), map BARs, negotiate features, set up the control
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virtqueue. `virtio-gpu-protocol.zig` for the control structs (host-tested sizes).
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- [ ] Create a 2D scanout resource backed by an `shm` region, `attach_backing`,
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`set_scanout` to CRTC 0, and `resource_flush` a test pattern.
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- [ ] Register a `scanout` service (new `ServiceId`).
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**Gate:** QEMU `-device virtio-gpu`: the driver comes up and logs
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`virtio-gpu: scanout {w}x{h} online`, and a flushed test pattern is visible (screenshot)
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— proving virtqueue + resource + set_scanout + flush end to end.
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## V4 — The native backend + hot-attach
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- [ ] `VirtioGpuBackend` in the compositor: `surface()` = the shared scanout resource,
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`present(damage)` = `resource_flush` of the damaged rect.
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- [ ] The driver **announces** to `.display` (looks it up, sends *attach-scanout* with its
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`scanout` endpoint + the shared surface as capabilities). The compositor switches
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backends and re-presents the current frame full-screen.
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- [ ] Boot still starts on `GopBackend`; the upgrade happens on announce.
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**Gate:** boot with virtio-gpu + `display-demo`; the compositor logs
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`display: scanout upgraded to virtio-gpu`, and the demo animates through the native
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backend (serial heartbeat + screenshot). Without `-device virtio-gpu`, it stays on GOP
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(the v1 gates still pass).
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## V5 — Mode-setting, EDID, and vsync
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- [ ] virtio-gpu `GET_EDID` → a mode list; `set_scanout` at a chosen mode = runtime
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resolution change. `runtime.display` gains `modes()` / `setMode(m)`.
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- [ ] A vsync/fenced `resource_flush` present path → genuinely tear-free.
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- [ ] The compositor reports the native backend's `canModeSet`/`hasVsync` = true.
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**Gate:** a `display-modeset` case changes resolution at runtime and confirms the new mode
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(`display: mode set to {w}x{h}`); a vsync present path is exercised without tearing
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artifacts in the frame counter/log.
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## V6 — Resilience (restart + re-attach) + tests + docs
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- [ ] The virtio-gpu driver is supervised (device-manager / init) and restartable; on
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driver loss the compositor freezes the last frame and **re-attaches** when the driver
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re-announces. Only a permanent give-up (crash-loop cap) attempts GOP again.
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- [ ] `test/qemu_test.py` cases: `virtio-gpu`, hot-attach, `display-modeset`, and a
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driver-kill/re-attach case. Update [display-v2.md](display-v2.md) status; README index.
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**Gate:** kill the virtio-gpu driver mid-run; the compositor survives and re-attaches on
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restart (`display: scanout re-attached`); all v1 + v2 cases pass; default `zig build` clean.
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---
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## Deferred (explicitly not in this plan)
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- **Client-rendered surfaces** — now unblocked by the `shm` capability (V2): an app renders
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its own bitmap and hands the compositor a reference. A natural follow-on.
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- **Bochs DISPI backend** — a simpler second native backend (mode-set only, dumb scanout);
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slots behind the same interface if wanted.
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- **Real-GPU (NVIDIA/AMD/Intel) drivers** — out of scope; those devices stay on the GOP
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floor by design.
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- **Hardware-accelerated compositing / multiple heads** — future.
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